The invention relates to a method for making a bipolar transistor providing protection against electrostatic discharges in an integrated circuit.
Integrated circuits are generally highly sensitive to electrostatic discharges. Indeed, these discharges result in excess voltages that are short-lived but have high values that may go up to several kilovolts in peak voltage, and induce instantaneous dissipated power values of several tens of Watts. These electrostatic discharges are likely to cause deterioration in the working of the integrated circuits and may even destroy these circuits, for example by excessive heating. It is therefore vitally necessary to take steps against this type of phenomenon by placing protective devices at the input of the integrated circuits.
A first type of protection consists of the connection of limiter diodes between input points of integrated circuits and supply terminals of these integrated circuits. This approach notably has the drawback of being restrictive for the threshold voltages of these diodes limits the range of the values permitted at these input points in normal operation.
Increasingly, therefore, a second protective device using bipolar transistors is being resorted to.
Let it be assumed, for example, that the protective device relates to an integrated circuit that is powered between a positive voltage and a ground and has one input, and that it is sought to protect this integrated circuit against electrostatic discharges of positive voltages.
The integrated circuit is protected by connecting an NPN type bipolar transistor between its input and the ground. The collector is connected to the input of the integrated circuit. The base and the emitter are connected to the ground. The transistor is therefore off in the absence of any electrostatic discharge. For, it is assumed that the voltage values permitted by the manufacturer at input of the circuit are lower than the avalanche voltage value of the protective bipolar transistor. In the event of electrostatic discharge, the protective transistor limits the excess voltage characterizing this discharge. Indeed, the excess voltage prompts an avalanche phenomenon in the protective transistor.
From the technological point of view, the integrated circuits are made notably by metal-oxide-semiconductor (MOS) technology.
With regard to the making of bipolar transistors for the protection of MOS type integrated circuits, it is advantageous to make bipolar transistors without superimposed layers. Indeed, the making of this type of structure requires more steps to be performed than the making of MOS type circuits. It is therefore desirable to integrate the manufacturing of the protective bipolar transistors into the process of manufacturing the MOS type circuits that they protect. This makes it possible to avoid having to add diffusion steps to those already required for the manufacture of a MOS type circuit. Consequently, lateral type bipolar transistors are preferably made for this use.
The method of manufacturing a lateral bipolar transistor of an NPN type in the example chosen typically includes the following basic operations:
the preparation of a P type substrate, PA1 the creation, on the surface of the substrate, of a field oxide layer by thermal oxidation, this layer demarcating two source/drain type implantation zones, PA1 the implantation of N type dopants in the implantation zones and the diffusion of these dopants in the underlying semiconductor substrate, PA1 the vapor phase deposition of an insulator layer on the circuit, PA1 the opening of contacts in the implantation zones, PA1 the metallization of the contacts. PA1 a semiconductor substrate implanted with a first type of dopant in a certain concentration, PA1 a field oxide layer covering a part of the semiconductor substrate, the uncovered part forming an active zone, PA1 two distinct implantation zones implanted with a second type of dopant in the active zone, PA1 a separation zone, in the active zone, between these implantation zones, PA1 a layer of an insulator deposited on the surface of the transistor after the implantation of the implantation zones, this layer of insulator being in contact with a part of the implantation zones and the separation zone, and wherein the deposited insulator is in contact with the semiconductor substrate at the position of the separation zone.
The implantation zones are zones known as active zones. An active zone is a surface zone of the substrate covered with thin oxide. An active zone such as this is therefore capable of being doped by implantation. Hereinafter, the term "implantation zone" will characterize the active zones that are actually implanted during the manufacturing process. In the example, the implantation or active zones, with their underlying N type diffusion, form the emitter and the collector (hereinafter called electrodes) of the bipolar protective transistor.
The base of the bipolar transistor is constituted by the substrate, the latter being connected to the ground. In practice, the useful base of the transistor is that part of the substrate that is located between the N type diffusions of the emitter and of the collector. The term "useful base" designates the base zone that comes into play in the transistor effect.
The implantation zones are demarcated by the field oxide on the surface of the substrate. Indeed, the field oxide counters the passage of the dopant particles. It therefore insulates the substrate during the doping operations. The implantation zones are characterized by the fact that they are not covered with field oxide.
The making of the implantation and of the diffusion of N type dopants in the implantation zone is integrated into the process for making a MOS type circuit. There is no stacking of three layers with successive N, P and N type doping.
It is possible, before the creation of the field oxide, to carry out a P type implantation and diffusion in the zones that will be covered by the field oxide. Thus, by diffusion, insulator walls known as channel stops are created between the emitter and the collector on the one hand, and between these electrodes and the rest of the circuit on the other hand. Since the substrate is initially a P type substrate, the concentrations of impurities in the insulation wells are naturally greater than that of the substrate.
The insulation wells enable the prevention of a piercing of the transistor, namely a passage of leakage current between the electrodes. Furthermore, these insulator diffusions diminish the formation of parasitic MOS type transistors whose gates would be formed by interconnections passing over the implantation zones.
This method has a drawback. Indeed, it has been observed that, after an electrostatic discharge, leakage currents could appear, in normal operation, between the collector and the base. The term "normal operation" is understood to mean operation that induces no conduction in the protective bipolar transistor.
It has been discovered that these leakage currents are due to the injection of hot carriers into the field oxide between the electrodes, similarly to the phenomenon of ageing of the MOS type transistors. This injection occurs in a region of the field oxide known as the "bird's beak". A bird's beak is a particular feature, shaped like a spike, located at the boundary of a field oxide and a surface of a thermally non-oxidized substrate. It is caused by the lateral oxidation of this substrate.
A hot carrier is the name given to a carrier (in this example an electron) accelerated by the field resulting from an electrostatic discharge and capable of acquiring sufficient energy to overcome the potential barrier of the field oxide. This injection occurs at the bird's beak placed at the boundary of the field oxide between the electrodes, and of the collector of the transistor. This injection remains limited as the carriers are hampered by the field oxide. An electrical field is created at the surface of the field oxide, on the collector side, and induces a leakage current between the collector and the substrate of the transistor.
In view of the above, the aim of the present invention is to propose a method for the making of a protective lateral bipolar transistor that can be used to eliminate the leakage currents that appear in normal operation, after a discharge.
The approach proposed by the invention is that of eliminating the field oxide zone which the injection of hot carriers occurs into, inducing leaks. This field oxide zone is the one between the two implantation zones corresponding to the electrodes of the bipolar transistor. It therefore covers the useful base of the bipolar transistor.
According to the invention, this aim is achieved by implementing a method for the manufacture of a protective bipolar transistor for protection against electrostatic discharges in an integrated circuit, comprising, in the following order, masking operations enabling the demarcation, for this bipolar transistor, on the surface of a semiconductor substrate, of two implantation zones and a separation zone between said implantation zones, and an operation for the implantation of the implantation zones with impurities enabling the selective doping of these implantation zones, wherein the masking operations comprise a first step with a total or overall contour masking operation to make a field oxide around an active zone encompassing the implantation zones and a second step with a masking enabling the separation, within this active zone, of the differentiated implantation zones, and wherein, subsequently to the operation for the implantation of the implantation zones, an operation is carried out to cover a part of the implantation zones and of the separation zone with a deposited insulator, this insulator being deposited against the surface of the semiconductor substrate at the position of the separation zone.
This thus prevents the injection of hot carriers into the bird's beak of the field oxide, at the separation zone of the implantation zones, on the collector side. The bird's beak has disappeared at this position.
The elimination of the field oxide between the implantation zones calls for steps to be taken with respect to the masks used during the operations for the thermal oxidation and doping of the implantation zones. Indeed, the demarcation of the implantation zones was previously obtained by the field oxide. The field oxide covered the separation zone between the implantation zones.
The method according to the invention makes it possible to dope the implantation regions selectively, by masking the separation zone temporarily during the doping of the implantation zones.
Thus, during the masking operations, there is made a field oxide that does not cover the implantation zones and the separation zone between the implantation zones. The separation zone between the implantation zones is masked temporarily during the operation for the implantation of the implantation zones, only the implantation zones being doped during this operation.
Hence there is no more than just one active zone (namely a zone covered with thin oxide) instead of two as was the case previously. This active zone includes the two implantation zones and the separation zone between said zones.
Usually, the implantation masks are generated automatically by a computer program, on the basis of the mask demarcating the active zones and information elements defining the type of doping (N or P) of these zones. It is therefore not usual to take action at these levels. Indeed, in MOS technology, the diffusions of a same type are either separated by a gate to form a MOS transistor or separated by field oxide.
The invention can be used to make a bipolar transistor providing protection against electrostatic discharges in an integrated circuit, comprising:
said device comprising
In standard MOS technology, the insulator deposited is in contact with MOS type transistor gates, field oxide zones and implantation zones.